Andreas Krimmer

In situ measurements of the spectral reflectance of metallic mirrors at the H

The efficient and reliable control and monitoring of the quality of the optical properties of mirrors is an open problem in laboratory plasmas. Until now, the measurement of the reflectance of the first mirrors was based on the methods that require additional light calibration sources. We propose a new technique based on the ratio of the red- and blue-shifted emission signals of the reflected hydrogen atoms which enables the in situ measurement of the spectral reflectance of metallic mirrors in low-density Ar-H or Ar-D plasmas. The spectral reflectance coefficients were measured for C, Al, Ag, Fe, Pd, Ti, Sn, Rh, Mo, and W mirrors installed in the linear magnetized plasma device PSI-2 operating in the pressure range of 0.01-0.1 Pa. The results are obtained for the Hα line using the emission of fast atoms induced by excitation of H atoms through Ar at a plasma-solid interface by applying a negative potential U = -80, …, -220 V to the mirror. The agreement between the measured and theoretical data of reflectance is found to be within 10% for the investigated materials (except for C). The spectra also allow us to efficiently determine the material of the mirror.

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Light-based diagnostic systems of fusion reactors require optical mirrors to channel light through the structures surrounding the plasma. With increasing plasma volume, power and plasma burn time, the environmental conditions grow more demanding and new requirements arise. In this dissertation, the design of optical mirrors inside the vacuum chamber of the prototype reactor ITER (Latin ”the way”) and future fusion power plants are investigated. Comparing the state of the art with the boundary conditions close to the fusion plasma, existing mirror designs and choices for the reflective surface are evaluated. For the design, it is not the individual boundary conditions that are critical, but rather, their combination and the resulting interactions. Drawing from the existing designs, possible realizations for central functionality are discussed. Included in the discussion are substrate choice, mounting, adjustment and thermal contacting as well as positioning of the mirror assembly compatible with hot cell maintenance. Building on the general discussion, mirror concepts for the charge exchange recombination spectroscopy (CXRS) diagnostic system for the ITER plasma core are proposed and simulated. In addition, prototypes are manufactured and tested to assess critical aspects of the proposed design. Testing includes positioning by pins, manufacturing of a stainless steel substrate with fluid channels adapted to the mirror shape, and tests with an SiO2/TiO2 dielectric coating under selected ITER conditions. As a result of the work, the fusion reactor mirror design considerations given in the principal design discussion can be used as a basis for other diagnostic systems as well. In the case of the core CXRS mirror concept for ITER, the basic suitability was shown and critical topics were identified where additional work is necessary.

line in a low density Ar–H plasma

Mirrors inside the ITER torus vacuum are used in many of the ITER port based diagnostic systems. As example, the core Charge Exchange Recombination Spectroscopy (core CXRS) concept is based on a mirror labyrinth of 7 consecutive in-vessel mirrors. For this system, ex-vessel alignment of up to three mirrors might be needed. Because of the unique and demanding environment in the ITER diagnostic port plugs, especially in the first meter from the plasma, existing designs of adjustable mirror holders cannot be used without modification. The wish to allow re-alignment of the mirrors during maintenance of the diagnostic system, if possible by remote-handling only, adds to the complexity. With many of the ITER diagnostics being in the stage of a conceptual design and advancing towards a more detailed design, the mechanical design of alignable mirrors themselves is starting. This paper discusses mirror alignment principles for use by ITER diagnostics. The core CXRS system is used as example throughout the paper. Loads and requirements for in-vessel diagnostic mirrors are discussed. Three basic groups of alignment principles are assessed for applicability in close proximity to the ITER plasma. The strength and weaknesses of the different alignment principles are discussed and a conclusion about their general applicability in ITER is given.